1. Field of the Invention
The present invention relates to an ink jet printing method and an ink jet printing apparatus, which form an image by forming a latent image with a colorless liquid and then making the latent image visible. This invention also relates to a recorded medium obtained from the above printing method and the printing apparatus, the recorded medium having the latent image.
2. Description of the Prior Art
In Japan a picture drawn in invisible ink which is later heated to make it visible has been known since long ago.
This kind of hidden picture is generally formed by drawing letters or pictures as a latent image on paper with a juice of citrus fruits and is made visible by heating the paper to turn the latent image to brown or black.
Such a conventional technique of turning a hidden picture to a visible form, or "aburidashi" as it is called in Japanese, is made possible because of the presence in the fruit juice of organic acid as a substance that develops color upon heating.
In realizing the conventional technique of "aburidashi" with the ink jet printing system, the following problems may be encountered.
(1) Ink used needs to contain organic acids, the substances that produce color. The organic acids in the ink, however, can attack ink tanks and head members. Hence, the use of organic acids is not desirable.
(2) Particularly when a so-called bubble jet system that employs an electrothermal transducer as an ejection energy generating element to cause film boiling to produce energy for ejecting ink is used to eject organic acid-laden ink for forming a latent image, there is a possibility of the organic acids scorching on a heater as the electrothermal transducer, which prevents heat of the electrothermal transducer from being transmitted to the ink efficiently, leading to unstable ejection of ink droplets usually called, "kogation problem".
(3) In addition to heads and ink tanks containing ordinary ink, the user must purchase other heads and ink tanks dedicated for the organic acid-laden ink. This will increase the running cost.
(4) The conventional technique of making a latent image visible requires heating. When, however, a print material on which a latent image is formed is flammable, the possibility of the print material catching fire cannot be ruled out and a problem remains in terms of safety.
There is also known a method that does not require heating to make a latent image visible. This method produces color by combining an acid substance or alkaline substance and a pH indicator such as litmus solution and phenolphthalein.
When this method is implemented with the ink jet system, however, the user needs to buy ink tanks containing the above chemical agents and associated heads in addition to the ordinary ink tanks. This will also raise the running cost.
Further, when this method is implemented with the bubble jet system, kogation caused by the chemical agents on the heater may depend on the kind of the agent, making it difficult for heat energy from the heater to be transmitted to the ink efficiently, which in turn leads to unstable ejection of ink.
As described above, in implementing these conventional color development techniques with the ink jet system or the bubble jet system, it is necessary to purchase special parts or components for forming a latent image and producing color and this increases the cost, which of course is not desirable for the user. With the bubble jet system, it is difficult to overcome the kogation problem with the heater.
The present invention has been accomplished to overcome the above-mentioned problems and provide an ink jet printing method and an apparatus for implementing the method, which perform printing by using ink for producing colors and an almost colorless liquid containing a processing liquid that makes colorant of ink insoluble or coagulate. More particularly, it is the object of this invention to provide an ink jet printing method and an ink jet printing apparatus, which employ a new color producing method or an image forming method--somewhat similar to the conventional latent image recovering technique--which involves first applying the colorless liquid to the recording paper to form a latent image and then applying the color ink to make the latent image visible.
Further, the present invention provides a recorded medium formed with a latent image of only the colorless liquid in a specified area thereof.